The purpose of this project is to continue our studies of mammalian renal medullary function and to pursue two broad objectives. The first is the urinary concentrating mechanism. Investigations of its three principal elements are planned. The loop of Henle will be studied in Psammomys to determine whether the mechanism of concentration in the descending limb is NaCl recycling or water extraction. The effect of antidiurectic hormone on the permeability and reflection coefficient of the collecting tubule to urea will be examined. Capillary permeability to macromolecules and blood flow in individual vasa recta under different conditions will be determined. Aside from these three elements, studies of the role of prostaglandins are also planned. The second broad objective is to study medullary recycling of potassium and its role in urinary potassium excretion. All studies will be performed in vivo. Those of the tubule will employ micropuncture, microperfusion and related microanalytical and related microanalytical techniques, including electron microprobe analysis. Investigations of vasa recta blood flow will also make use of television microscopy. In several studies, mathematical models will be formulated.